China did not, and still doesn't, want US troops at its border. That's why it originally intervened and why it supports North Korea. At the time there was also a further risk that the US might invade China.

That doesn't have to be the result of it. A more humane regime in NK doesn't mean reunification has to happen. And, part of the reason those US forces are in South Korea is the threat of the North. By threatening US involvement in case of an attack.